This invention relates, in general, to reactive ion etching, and more specifically, to reactive ion etching for surface acoustic wave (SAW) devices.
SAW devices can be designated to provide several signal processing functions such as band pass filtering time delay, compression/expansion, and resonator functions. These SAW devices are frequently utilized in mobile communication units.
The acoustic signal wavelength of the SAW and the transducer feature size are inversely proportional to the RF signal frequency. As the frequency of the signal increases, the seize of the transducer must decrease. Transducers in the microwave frequencies become very small, and the line and space widths are accordingly small.
Conventional methods for fabricating very small transducers use wet chemical etching. A conductor layer is first formed on a wafer substrate, and a photoresist layer is placed on the conductor layer. The photoresist layer is then developed to define the pattern for the conducting metalization layer. The transducer on the wafer is then etched using a wet chemical etchant.
Wet chemical etching conventional methods cannot produce very small electrodes needed for very high frequency ranges. The higher the operating frequency range, the less likely the wet etchant process can fabricate the widths of the electrodes needed. Electrode sizes for frequencies in the microwave range are virtually impossible using conventional wet etching. The reason the wet etching cannot fabricate very fine electrodes is because the wet etchant corrodes in all directions at the same time. Therefore, rather than only etching down to remove the metal between the electrodes, the wet etchant etches into the electrodes themselves.